The concentration dependence, kinetics, and mechanism of the restorative effect of normal human immunoglobulin (Ig)G on deficient serum opsonization for serotype 10 Streptococcus pneumoniae in children and adolescents with sickle cell disease will be investigated. Labeled bacteria will be incubated with unsupplemented patients' sera or patients' sera supplemented with increasing concentrations of purified normal human IgG. The bacteria will be washed, incubated with normal human polymorphonuclear leukocytes (PMNs), and the kinetics of uptake and intracellular killing will be determined. Results will be compared to those obtained with bacteria opsonized with age-matched control sera. Subsequent experiments will be performed to determine if the restorative effect of IgG is due to enhancement of alternative or classical complement pathway C3 convertase formation with resultant cleavage and surface deposition of trace-labeled C3 or augmentation of uptake by the leukocytes. Binding of labeled F(ab')2 fragments, Fc fragments, and whole IgG to the pneumococci will be compared, and the restorative effect of the fragments on the deficient opsonization will be determined. Relationships between opsonization for multiple pneumococcal serotypes and opsonization and alternative complement pathway activation by the homologous serotype will also be investigated. Uptake by human PMNs of labeled serotypes 7, 10, 15, and 24 after opsonization with patients' or control sero will be measured. Alternative pathway-mediated reduction in the B antigenic determinant of human C3 will be quantitated in the sera after incubation with the bacteria. Data from the patients and controls from each measurement will be compared, and specifide correlations on the patient's data will be determined. The knowledge derived from this investigation will increase understanding of the cause of deficient pneumococcal serum opsonization in the sickle cell disease and of one mechanism by which this abnormality can be corrected.